This invention relates to air separation.
Modern chemical and metallurgical processes including an oxidation step call for ever larger quantities of oxygen. Oxygen can be produced in quantities in excess of 2,000 tonnes per day by an air separation process which comprises compressing an air stream, purifying the air stream by removing therefrom components of relatively low volatility such as water vapour and carbon dioxide, cooling the thus purified air stream to a temperature suitable for its separation by fractional distillation or rectification, and then performing that separation so as to produce oxygen product of desired purity. The purification is preferably performed by beds of adsorbent which adsorb the components of low volatility such as water vapour and carbon dioxide. The fractional distillation or rectification of the air is preferably performed in a double rectification column comprising a higher pressure stage and a lower pressure stage that typically share a heat exchanger effective to condense nitrogen at the top of a higher pressure column and reboil oxygen-rich liquid at the bottom of the lower pressure column. Some of the thus formed liquid nitrogen is used as reflux in the higher pressure column while the remainder is typically removed from the higher pressure column, is sub-cooled, and is passed through an expansion valve into the top of the lower pressure column so as to provide reflux for that column, the air being introduced into the higher pressure column. Oxygen-enriched liquid air is withdrawn from the bottom of the higher pressure column and is passed to the lower pressure column where it it typically separated into substantially pure oxygen and nitrogen products. These products may be withdrawn from the lower pressure column in the gaseous state and warmed to ambient temperature in countercurrent heat exchange with the incoming air, thereby effecting the cooling of the incoming air. Since such a process operates at cryogenic temperatures, refrigeration has to be generated. This is typically done either by expanding a part of the incoming air in a turbine or by taking a stream of nitrogen from the higher pressure column and passing it through an expansion turbine.
In DE-A-2 854 508 there is disclosed a process in which all the air exiting the turbine is introduced into the lower pressure column. Such a process offers the advantage that introduction of the turbine-expanded air into the lower pressure column helps to enhance the thermodynamic efficiency with which rectification takes place when there is not a requirement to produce either a liquid oxygen or a liquid nitrogen product. The process is however limited in that it is generally not suitable for use when it is desired to produce liquid oxygen and/or liquid nitrogen products (in addition to a gaseous oxygen and/or gaseous nitrogen product) in a total amount of more than 5% of the gaseous oxygen product.
It is also known to provide refrigeration for an air separation process by arranging for an air expansion turbine to exhaust into a higher pressure column. Such a process therefore entails compressing the air to a higher pressure than those at which the double rectification column operates. Such a process is disclosed in U.S. Pat. No. 2,779,174. By arranging for the expansion turbine to exhaust into the higher pressure column, it is possible to obtain a greater yield of liquid oxygen or liquid nitrogen than in the process described in DE-A-2 854 508. Our analysis shows however that the process is relatively inefficient thermodynamically, especially when the liquid production is in the range of 10 to 50% of the plant output.
A large number of proposals also exist in the art for using two expansion turbines in parallel with one another to generate refrigeration for the process and to enable any desired proportion of the oxygen and nitrogen products to be produced in liquid state. An example of such a proposal is given in U.S. Pat. No. 4,883,518. In general, in comparison with single turbine systems, additional passes are required through the heat exchanger in which the air is cooled. Moreover, such processes employing two turbines in parallel require a large proportion of the turbine-expanded air to be directed other than to the lower pressure column.
EP-A-0 420 725 discloses an air separation cycle in which a part of a main compressed air stream is withdrawn from a main heat exchanger at a first intermediate location; is expanded in a first turbine; is returned through the heat exchanger from its cold end to a second intermediate location at a higher temperature than the first intermediate location; is withdrawn from the second intermediate location is expanded in a second turbine, which therefore operates at higher temperatures than the first turbine; and is then mixed with an impure nitrogen stream flowing through the main heat exchanger from its cold end to its warm end. The entire air stream is compressed to a relatively high pressure in the order of 30 bar and all the oxygen product is produced as liquid.